(1) Field of the Invention
The present invention relates to a post-processing device, particularly to a post-processing device that punches a hole in a sheet on which an image is formed by a printer, a copying machine, or the like and an image forming system including a combination of an image forming device and the post-processing device.
(2) Description of the Related Art
A post-processing device executing processing such as punching of holes in a sheet by using punching rods and stapling sheets by using a stapler is configured to receive sheets on which images are formed and which are successively fed from an image forming device such as a printer and a copying machine. When denoting a center of a sheet along a sheet width direction (hereinafter referred to as a “CD direction”) orthogonal to the conveyance direction as a “sheet center” and a center of a sheet conveyance path along the CD direction of the post-processing device as a “sheet conveyance center”, the sheet center of a sheet that is fed to a post-processing device from an image forming device sometimes is to some extent shifted from the sheet conveyance center, although the sheet center is supposed to pass the sheet conveyance center during post-processing. When a hole puncher of the post-processing device punches holes in a sheet whose sheet center is shifted from the sheet conveyance center, the holes inevitably are not axisymmetric with respect to the sheet center.
In view of this, in conventional post-processing devices, a shift of the sheet center from the sheet conveyance center along the CD direction is detected, the hole puncher is moved so that a center of the hole puncher matches the sheet center, and hole-punching is executed. In such a structure, the shift of the sheet center from the sheet conveyance center along the CD direction is detected by sheet end detection sensors. Each of the sheet end detection sensors includes a light-emitter and a light-receiver, and a light-emission intensity of the light-emitter is adjusted to a preferable value. In order to adjust the light-emission intensity of the light-emitter, light of various light emission intensities is emitted from the light-emitter and an amount of light reflected on a surface of a mirror facing the sheet end detection sensors is measured. However, when a sheet is remaining at the hole puncher during adjustment of the light emission efficiency of the light-emitter, the light-emission intensity of the light-emitter is adjusted erroneously because an amount of light reflected on the sheet is measured. In conventional post-processing devices, whether or not a sheet remaining at the hole puncher (hereinafter referred to as a remaining sheet) is existent is detected by using sheet sensors disposed upstream in the conveyance direction relative to the hole puncher and downstream in the conveyance direction relative to the hole puncher. The adjustment of the light emission intensity is executed after determination that no remaining sheet is existent at the hole puncher is made by using the sheet sensors.
In recent years, there is a demand for post-processing of sheets in various sizes. Conventional devices configured to achieve such post-processing, however, cannot detect sheets having small sizes remaining between the sheet sensors. However, disposing another sheet sensor for detecting such remaining sheets leads to high cost, and therefore is not desirable.